Heretofore, switching power supply circuits that are included in various electronic apparatuses as a power supply have been proposed (refer to JP 2001-339948 A, hereinafter referred to as “Document 1”, for example).
A switching power supply circuit described in Document 1 includes a full-wave rectifying circuit including a bridge rectifying circuit and a smoothing capacitor, and a main switching element. Also, the switching power supply circuit includes a switching driving unit configured to drive the main switching element, and an insulated converter transformer that includes a primary winding and a secondary winding.
A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is used as the main switching element. A parallel resonance capacitor is connected between a drain and a source of the main switching element. Also, a clamp diode is connected between the drain and source of the main switching element.
An active clamp circuit is connected in parallel to the primary winding. The active clamp circuit includes an auxiliary switching element, a self-excited driving circuit configured to drive the auxiliary switching element, a clamp capacitor, and a clamp diode. The self-excited driving circuit includes a drive winding that is provided in the insulated converter transformer, a capacitor, and two resistors.
A voltage doubler half-wave rectifying circuit is formed on a secondary side of the insulated converter transformer. The voltage doubler half-wave rectifying circuit includes a secondary winding, a serial resonance capacitor, a first rectifying diode, a second rectifying diode, and a smoothing capacitor. The serial resonance capacitor is connected in series to the secondary winding. The first rectifying diode is connected in parallel to a series circuit of the secondary winding and the serial resonance capacitor. A series circuit of the second rectifying diode and the smoothing capacitor is connected in parallel to the series circuit of the secondary winding and the serial resonance capacitor.
As described above, since the voltage doubler half-wave rectifying circuit is formed on the secondary side of the insulated converter transformer in the conventional switching power supply circuit described in Document 1, it is difficult to further downsize the switching power supply circuit. Also, it is desired recently that power supply devices that are included in various electronic apparatuses as a power supply is further downsized.